The present invention relates to wireless superheterodyne receivers and, more particularly, to superheterodyne receivers which have a crystal filter connected to the output of a preamplifier for filtering out undesired frequencies and a demodulator having a single mixer supplied by an oscillator having a second crystal, the oscillator signal mixing with the received signal for yielding a desired intermediate frequency.
Generally, wireless receivers come in two basic varieties, superregenerative and superheterodyne. The superregenerative receiver, which may for example consist of a simple one transistor RF receiver, has the advantages that it consumes very little power and can be made extremely sensitive. However, it is not an optimum choice in terms of noise rejection and temperature drift, and it can radiate broad band RF energy back out of the antenna.
By contrast, superheterodyne receivers are typically more complex, consuming greater amounts of power. They typically require two mixers for their operation which increases complexity and power usage. Such receivers, however, can find many uses if they can be made small, if they do not suffer from temperature drift problems, if they do not radiate broad band RF energy back into the antenna and if they can be made to reject noise.
For example, in today's building automation systems, a central station is connected over a transmission channel to a plurality of remote stations which are then wired to various sensors and control loads. The remote stations can be made somewhat independent of the central station by including intelligent processors for performing tasks on their output loads based upon inputs that they receive from their sensors, other tasks of the control system being performed by the central station.
Installation complexity and cost can be greatly reduced if the sensors and control loads, which are normally wired to the remote stations, can communicate with the remote stations instead by wireless transmissions. Since it may be desirable for certain of these sensors and/or control loads to be battery operated, it is necessary that the wireless receivers, which would have to be included in the control loads and/or sensors, use a minimum amount of power. Furthermore, the size of the receiver must be small thereby necessitating a simple receiver and the receiver must be designed for optimum noise rejection, minimum temperature drift and minimum radiation of the broad band RF energy back out of the antenna. A single mixer superheterodyne receiver can be made to fulfill these requirements operated at very low power levels.